Glutathione peroxidase has been found to be an additional component of the hepatic phenylalanine hydroxylase system, providing protection from the deleterious effects of hydrogen peroxide. Absence of carbinolamine dehydratase activity may result in hyperphenylalaninemia and conversion of 6-biopterin to 7-biopterin. Contrary to published work, inhibition of dihydropteridine reductase by aluminum is not a factor in dialysis encephalopathy.